Effects of Flow on Protective Scales Formation in a CO2 Saturated Brine Environment

Abstract

A significant amount of literature exists demonstrating the effect of CO2 on corrosion scale formation under non-flow conditions and its pH and temperature dependent protectivity. However, little information is available on the effects of flow on the nature and protectivity of corrosion scale. This work demonstrates the effects of flow on 4 different oil-field grade steels of varying chemistry (in terms of micro-alloying) and microstructures (ferritic /pearlitic and martensitic), in a CO2 saturated (1bar) brine (0.5 M NaCl) environment under conditions (pH = 6.3 and temperature = 80 oC) which generate a significantly compact and protective scale under non-flow conditions. Flow was introduced in the range of 0 - 3000 RPM via a rotating disk electrode (RDE) type arrangement which has been customized to comply with the needs of the experiment. The effect of flow was electrochemically characterized by potentiostatic current density (id ) vs time (t) measurement where Pt and Ag/AgCl were used as counter electrode and reference electrode respectively. Higher RPM (≥1000) indicated distinct characteristics compared to the lower RPMs (≤100) with a transition of i vs. t behavior observed around 500 RPM. Higher RPMs seemed to modulate the mechanism of the nucleation stage of scale formation and induce a relatively sluggish formation of protective scale by delaying the crystal growth process. Two plain carbon steels, J55 (ferritic /pearlitic) and L80 (Martensitic), did not show any qualitative difference in their i vs. t characteristics as function of flow. However, L80 steel required a longer time for crystal growth and achieving a similar degree of protection through scale formation compared to J55. 1 wt.% Cr micro-alloyed steels showed a lower current density compared to the plain carbon steels at the very early stage of scale formation; however the eventual protectivity of the scales formed on the plain carbon steels appeared to be much better on the basis of i vs t characteristics. Further characterizations of the scales in terms of phase identification, morphology and thickness by XRD, SEM and profilometry are in progress. Figure 1